The MRI converts spatial distribution of information on relaxation of proton density and nuclear magnetic resonance signals (hereinafter referred to as NMR signals) into an image so as to obtain form information or functional information of the head, abdomen, limbs, etc. of a human. The nuclides to be imaged with the MRI which are widely used in clinical practice at present are hydrogen nuclei (protons) that are main components of a subject.
For measuring a human body, examples of main substances from which NMR signals from protons can be detected with the MRI include water and fat. For converting NMR signals from water protons (hereinafter referred to as water signals) into an image, the image contrast of NMR signals from fat proton (hereinafter referred to as fat signals) is lowered. In clinical practice, a method for suppressing in vivo fat signals is therefore proposed.
An example of the technique for suppressing signals from unnecessary substances other than the substances to be imaged is the Dixon's method in which the difference in resonance frequency between substances are used (refer to NPL 1, for example). The Dixon's method separates signals of respective substances with use of the resonance frequency difference so as to obtain an image in which signals of unnecessary substances are suppressed. The resonance frequency difference is called a chemical shift, and the resonance frequency difference between water and fat is 3.5 ppm, for example. The resonance frequency difference that is a chemical shift is proportional to magnetic field strength, and is approximately 448 Hz when the magnetic field strength is 3 teslas.
The Dixon's method obtains an image from each of a plurality of NMR signals (hereinafter referred to as echo signals) that are different in the time (hereinafter referred to as the echo time) from excitation of hydrogen nuclear spins until acquisition of the echo signal. There are methods called the two-point Dixon (2PD) method and the three-point Dixon (3PD) method depending on the number of echo times to be measured.
In the 2PD method, images obtained from echo signals measured at two different echo times are used for estimation of an offset frequency distribution. The offset frequency distribution is a distribution of offset amounts of resonance frequencies varying spatially owing to inhomogeneous magnetostatic field. This is then used for calculation of an image (hereinafter referred to as a water image) in which the strengths of water signals are pixel values and an image (hereinafter referred to as a fat image) in which the strengths of fat signals are pixel values, for example (refer to NPL 2, for example). In the 3PD method, images obtained from echo signals measured at three different echo times are used for calculation of a water image, a fat image, and an offset frequency distribution (refer to NPL 3, for example).
Furthermore, there is also a technique called the Multi Point Dixon (MPD) method in which images at four or more echo times are measured, and a water image and a fat image, an offset frequency distribution, and an apparent transverse magnetization relaxation rate (R2*) distribution are calculated (refer to NPL 4, for example). The MPD method is used in a super-high magnetic field MRI of 3 teslas or higher with greatly inhomogeneous magnetostatic field and transmission magnetic field and in a fat suppressing method for an abdomen, a cervical spine region, etc. The MPD method is also used for quantification of fat signals, evaluation of hepatic steatosis diseases through identification of iron deposits in a liver, etc.